This invention relates to fluid-filled transducers, and, more particularly, to fluid-filled transducers for making pressure measurements.
Fluid transducers are widely used in measuring physical phenomena. A typical example is the measurement of cardio-vascular pressure using fluid-filled catheters. While fluid-filled catheters do not have as desirable a frequence response as, for example, the catheter-tipped manometer, they are less fragile, less costly and less difficult to calibrate. As a result fluid-filled catheters continue to be in widespread use.
However, fluid-filled catheters produce measurements with distortions which are directly related to the physical characteristics of the catheter.
In order to provide reliable readings with fluid-filled catheters, it has been necessary to compensate for the physical characteristics of the catheter. This has been done in a variety of ways.
One technique used in compensating for the physical property of the catheter has been to use mechanical damping, as disclosed, for example, by A. C. Lapointe and F. A. Roberge in "Mechanical damping of the manometric system used in the pressure gradient technique", IEEE Transactions, Biomed. Eng. 21:76, 1974; and by R. B. Jennings Jr. and L. J. Krovets in "The use of a damping chamber and sine wave oscillator for optimal frequency response in pressure recording", In IEEE Transactions, Ind. Elec. Con. Inst. 17:134, 1970.
Another technique has been to use low pass filtering of the kind disclosed by K. L. Gould, S. Trenholmn and J. W. Kennedy in "In-vivo comparison of catheter mamometer systems with the catheter-tip mamometer", J. Appl. Physiol. 34:263, 1973, and by H. E. Dear and A. F. Spear in "Accurate method for measuring dP/dt with cardiac catheters and external transducers", J. Appl. Physio. 6:897, 1971.
Another approach has been to use analog techniques and digital techniques to provide the inverse of the catheter transfer function. Examples of the inverse analog techniques are disclosed by A. Damenstein, R. L. Stout, H. U. Wessel and H. M. Paul in "Electronic compensator for pressure waveform distortion by fluid-filled catheters", Med. & Biol. Eng., March 1978; by J. Melbin and M. Spohr in "Evaluation and correction of manometer systems with two degrees of freedom", J. Appl. Physiol., No. 5, November 1969; and H. L. Falsetti, R. E. Mates, R. J. Carroll, R. L. Gupta and A. C. Bell in "Analysis and correction of pressure wave distortion in fluid-filled catheter systems", Circulation, Vol. XLIX, January 1974.
The inverse-digital technique has been disclosed by L. J. Krovets, R. B. Jennings and S. D. Goldbloom in "Limitation of correction of frequency dependent artefact in pressure recordings using harmonic analysis", Circulation, Vol. 50, November 1974, and by S. Cicolella, L. Most, L. Jackson and D. Jaron in "Compensation of Fluid-Filled Catheter Response using Digital Filter Techniques" in PROC 4th N.E. Bioengineering Conference (May, 1976), New Haven, Connecticut.
All of the foregoing techniques have several major drawbacks. They all require pre-calibration before the instrument is placed in use. This calibration is difficult to perform in a clinical setting and the catheter response is frequently different when it is used in a patient. Consequently the compensation that is afforded by the technique, when based on a calibration that does not actually apply in a clinical situation, will be incorrect. In addition during the long term monitoring of blood pressure, for example, slow changes in the catheter response may occur. None of the foregoing compensation techniques can take into account these slow changes since they are based upon prior calibration which takes place before the instrument is placed in use, and are instead based upon a laboratory response to a test "pop" signal.
Accordingly, it is an object of the invention to facilitate the use of fluidic transducers, particularly fluid-filled catheters which are used in measuring pressure such as that exerted by the cardio-vascular system.
Another object of the invention is to compensate for the physical characteristics of fluidic transducers. A related object is to compensate for physical characteristics without the need for mechanical damping, analog low pass filtering, or the use of inverse transducer functions on an analog basis.
A further object of the invention is to avoid the need for pre-calibration of fluidic transducers in the laboratory prior to actual clinical use. A related object is to achieve compensation of fluidic transducers in the actual clinical setting.
Still another object of the invention is to avoid the need for pre-clinical calibration in clinical situations. This is to avoid the possibility that conditions in the clinical setting may be different than those of the pre-clinical calibration with the result that a prior calibration may be inappropriate to a clinical situation.
Still another object of the invention is to achieve compensation in fluidic transducers, particularly fluid-filled catheters, to compensate for long term monitoring of physical phenomena, such as blood pressure. This allows slow changes in the response of the device to be corrected without requiring any further calibration or without requiring periodic interruptions in the long term monitoring in order to check the prior calibration.